By: Adam Baker
Manage DC voltage, wash your panels, and double the life of your bearings by simply integrating your solar panel trackers into your SCADA system.
One of the obstacles preventing a solar power plant running as a fine-tuned machine is that all equipment is developed independently, with the intent of running independently. An inverter starts in the morning, runs at maximum output as long as it can, and shuts off when the sun goes down. It doesn’t care if trackers are working, or the status of DC health. Trackers pay no attention if the inverter they’re associated with isn’t running. They are solely focused on following the sun.
My background in automation helps me see the value in connecting standalone equipment so they can work better together. In PV solar, there are several opportunities to optimize output by integrating equipment. With a small amount of integration there are a few basic improvements you can do to optimize trackers.
1. Preserve actuator lifespan
If an inverter is offline for maintenance, or curtailed offline, why would trackers continue to follow the sun? (Unless the outage is expected to be very short.) Why not move them to a stow location?
I’ve seen trackers uselessly follow the sun for a couple months after a fire in a recombiner rendered the inverter unusable. The thing that makes trackers so expensive to maintain is the wear and tear of moving parts. Why then, did this site waste months of tracker movement for absolutely no output? When I asked the owner why, he simply stated they didn’t have an easy way to turn their trackers off, and then back on later.
Obviously, some simple SCADA integration would have helped here.
2. Avoid overvoltage
DC overvoltage is happening less frequently these days, as inverters have become more robust than when I started in the industry. However, sites with space constraints and high DC/AC ratio arrays can still run into this problem.
On sites with very high DC/AC ratios, you can prevent inverters tripping on DC overvoltage by monitoring the DC voltage coming into the inverter, and integrating tracker commands. If an inverter’s DC voltage input is nearing the high operating limit in which the inverter would trip or curtail, you can set up an automatic command that tells trackers to move the panels forward, away from the sun. This reduces DC voltage temporarily, but allows the inverter to maintain maximum power output. The DC voltage will rise as the earth’s rotation moves the sun closer to perpendicular to the modules.
Tracker integration can keep the site operating in conditions when inverters might otherwise trip offline.
3. Wash your panels for free
Taking advantage of rain to get more soiling off panels is one of the easiest controls improvements on a solar farm.
The plant controller knows the weather from the met station. It also knows the generation output of the plant. The controller could be used to command the trackers to a preset location that optimizes cleaning.
If it’s raining and generation is low, you can set your trackers to automatically point to a near flat position. The position will have enough angle for the water to run off and carry any accumulated dirt and debris with it. You may lose a little energy, but if it’s raining, you’re likely not making the maximum amount anyway.
The potential gains from the increased output when sunny may offset the cost of operating sub-optimally in the rain in as little as 10 minutes.
4. Reduce wear and tear with fewer movements
If you think you’re optimizing your plant by keeping the panels pointed more accurately at the sun, you might want to recalculate.
Every time a tracker transitions from stationary to moving, each bearing has to overcome sticktion. Sticktion is the force required for a nonmoving object to move. It actually takes more force to overcome sticktion, than it does to move the object.
Someone not intimately familiar with the physics of solar and tracking might think that moving solar panels more frequently to stay directly under the sun is a good thing. In fact, the opposite is true. Most trackers move about every 10 minutes, which is much faster than necessary.
The angle between the sun and a solar panel tells us that if you’re within 3 degrees of pointing toward the sun, you’re at 99.9% of available energy (Sine of 87 degrees). The sun moves 15 degrees across the sky every hour, so moving every 24 minutes will keep you at 99.9% optimal DC power.
But when you bring in clipping, movement becomes even less necessary. Because tracking sites still have DC/AC ratios greater than 1.0 (but less than the typical 1.3 at fixed sites), they already have more DC power than can be handled by the available AC power limit. Even with inverter efficiency losses, the inverter is clipping when DC is >~95% of optimal position.
Which begs the question…if the maximum you can produce is 95% of what’s available, why are you trying to keep trackers at 99.9%?
It’s an easy argument that moving every 40, 50, or even 60 minutes will still keep you above the 95% of available DC. As an added bonus, moving less frequently means less sticktion events per day. Less sticktion events will extend bearing life and reduce the amount of energy (however small) consumed by the tracker system in its move events.
5. Save money on vegetation management
This is a somewhat obscure, but convenient use for integrated tracker controls. If you don’t have sheep or goats on your property, you’ve probably contracted with a vegetation management company. I’ve seen people trying to mow under a panel when it’s tilted…and it looks pretty difficult. Having a mode on your trackers that points them flat will make it a lot easier for vegetation management personnel to get around.
The more work they have to do to avoid the panel, the more they charge you. If panels are flat and easy to get under, you could most likely cut a deal with the company to save a little on site maintenance.
6. Pre-empt catastrophic wind events
Most tracker manufacturers have a wind-stow mode. If wind sensors around the site detect a high wind speed, trackers will automatically move to a near-flat position to prevent mechanical failure of the structural support. The problem is, this process is binary. Either the wind is 60kph, or it’s not. Wind sensors can’t predict the weather.
Integration with SCADA, however, may allow you to know that in 20 minutes, a severe storm is predicted to roll through. The weather front may include gusty winds that are beyond your mechanical design limits.
Controls integration could take this predictive information, and move your trackers closer to stow position in anticipation of strong winds. This would allow more time to ‘get safe’ when the microburst gets to the site, and anemometers start to issue the ‘stow’ command.
In summary, trackers do what trackers do, and 95% of the time they are fine doing that. The 5% of the time you wish they would do something a little different has a small opportunity cost relative to energy, but the potential to return many times that value later on. Tracker integration with SCADA and controls isn’t hard, it’s just uncommon. It doesn’t matter which tracker you use, they all can benefit from a little coordination with onsite infrastructure.
Adam Baker is Senior Sales Executive at Affinity Energy with responsibility for providing subject matter expertise in utility-scale solar plant controls, instrumentation, and data acquisition. With 23 years of experience in automation and control, Adam’s previous companies include Rockwell Automation (Allen-Bradley), First Solar, DEPCOM Power, and GE Fanuc Automation.
Adam was instrumental in the development and deployment of three of the largest PV solar power plants in the United States, including 550 MW Topaz Solar in California, 290 MW Agua Caliente Solar in Arizona, and 550 MW Desert Sunlight in the Mojave Desert.
After a 6-year stint in controls design and architecture for the PV solar market, Adam joined Affinity Energy in 2016 and returned to sales leadership, where he has spent most of his career. Adam has a B.S. in Electrical Engineering from the University of Massachusetts, and has been active in environmental and good food movements for several years.